Vaughan
multiplex signaling system



Nov. 23, 1965 H. E. vAuGHAN Re 25,911

MULTIPLEX SIGNALING SYSTEM Original Filed Sept. 11, 1958 2 Sheets-Sheet1 Nov. 23, 1965 H. E. vAUG-HAN MULTIPLEX SIGNALING SYSTEM 2,Sheets-Sheet 2 Original Filed Sept. 1l 1958 bm EN Hmmm k @Sm t, m, bm,@mmm @A k l@ United States Patent Otlce Re. 25,911 Reissued Nov. 23,1965 25,911 D'IULTIPLEX SIGNALING SYSTEM Henry E. Vaughan, Colts Neck,NJ., assigner to Bell Telephone Laboratories, Incorporated, New York,N.Y., a corporation of New York Original No. 3,950,589, dated Aug. 21,1962, Ser. No. 760,430, Sept. 11, 1958. Application for reissue Apr. 2S,1963, Ser. lo. 276,143

17 Claims. (Cl. 179-18) Matter enclosed in heavy brackets L' 3 appearsin the original patent but forms no part of this reissue specification;mntter printed in italics indicates the additions made by reissue.

The present invention relates to signaling systems and more particularlyto selective signaling means for use in communication systems operatedin time division multiplex.

In order to consolidate equipment and conserve communication paths in atelephone system, considerable effort is being directed to thedevelopment of systems utilizing time division rather than, or inconjunction with, space division as heretofore exclusively employed.Essentially, in space division each connection between two telephones isestablished over a distinct transmission path, While in time division asingle transmission path may be shared by a plurality of pairs ofcommunicating parties. The latter operation is accomplished by samplingeach conversation in turn at a high repetition rate, the speech samplesbeing fi tered at the terminals to produce a remarkably accuratefacsimile of the transmitted intelligence.

Further refinement is realized by the introduction of pulse codemodulation of the sampled intelligence to ease transmission requirementsof the system. Such a system, incorporating time division samplingtechniques and pulse code modulation of the sampled intelligence fortransmission, is disclosed in D. B. James, J. D. Johannesen, M. Karnaughand W. A. lvlalthancr Patent No. 2,957,949, issued October 25, 1960.

The system disclosed in the above-cited application also incorporates acentralized control type of operation such that signaling equipmentadvantageously is located at the central control point for service tothe entire system including a plurality of remote conccntrator units.This expedient obviates the duplication of ringing equipment at each ofthe remote concentrator units.

A number of distinct signaling indications including ringing tone, busytone, dial tone, etc., are required in such a system. As these signalingindications must pass over thc common transmission link between thecentral control source and the calling and called parties, they too mustbe sampled on a time division basis and the samples placed in codedform. It is essential that each signaling indication be available onrequest to every Station in the system in the sampling time allotted tosuch station.

The central control itself is not equipped to encode speech samplessince this function is performed exclusively at the remote concentratorunits, the central control serving merely to switch the coded samplesthrough to the proper termination. Thus, in order to supply cachtelephone in the system with any one of a plurality' of different tonesoriginating from a central control point. it is necessary to providecoding equipment at the central point so as to place each tone in Codedform for transmission to the outlying stations.

The provision of distinct coding equipment for each desired tone is bothuneconomical and inefficient. In such an arrangement a coder per tonewould provide a distinct coded sample of the preassigncd tone in eachsampling interval or time slot in a recurrent cycle of time slots. It isprobable that each tone would be required by the system in only a smallfraction of the total time Ll l) slots, so that during the remainder ofeach cycle, each coder would be operating needlessly.

It is an object of this invention to provide an improved time divisionmultiplex signal coding circuit.

It is another object of this invention to provide an economical signalcoding circuit which utilizes the maximum potential of the codingapparatus.

1t is a further object of this invention to provide a single codingcircuit which will furnish a plurality of coded signals simultaneously.

These and other objects of the invention are attained in one specificillustrative embodiment wherein a tone coding circuit comprises a singlecoder of a type known in the art in which samples of low frequencysignals are placed in digital or binary code form. One form of codersatisfactory for this purpose is that disclosed in J. R. Pierce Patent2,451,044, issued October l2, 1948. The tone signal samples are gatedsequentially into the coder which, in turn, translates the samples intodigital form and gates them sequentially to output terminalscorresponding to the signal sources.

A time shared telephone system of the type described shares a commontransmission link among a number of simultaneous conversations bydividing a prescribed time interval, known as a frame, into a pluralityof sampling intervals known as time slots. Each active line is assigneda time slot and is sampled in that time slot during successive frames.Sampling at a sulliciently high rate permits accurate reproduction ofthe original speech at the receiving terminal. An acceptable frame rateis eight kilocycles or twice the highest speech frequency to betransmitted. With this frame rate each conversation is sampled onceevery llama of a second or microseconds. Assuming 25 time slotsavailable in the frame, the interval of each time slot is l/ZSXSOUGth ofa second, or 5 microseconds. Each conversation then is sampledrepetitively during a distinct 5 microsccond time slot in every 125microsecond frame.

In such a telephone system each of a plurality of tones must beavailable for instantaneous assignment in any one of the time slots ofthe frame. With the low frequency tones employed, a new tone sample perframe interval is sutilcient for the system to reproduce the originaltone. Thus at the eight kilocycle frame rate of the above example, eachcycle of a 400 cycle tone would be sampled twenty times, an amount whichwill permit adequate reproduction of the original tone by the system,However, this tone sample must appear in each time slot of the frame inorder to satisfy the systems requirement that tones be available forinstantaneous assignment in cach idle time slot.

The employment of a single coder to process samples from a plurality oftone sources sequentially in each fraaie presents a problem in that itpermits the appearance of cach tone at a corresponding output terminalfor only a single time slot in the frame. A new sample of the same tonewill appear at the corresponding output terminal during the same timeslot in the next succeeding frame, but during the balance ofthe frame,there is no coded sample at the output terminal as required by the timeshared telephone system.

This ditticulty is overcome, in accordance with this invention, by theprovision of circulating delay means at each output terminal. In thisfashion a coded sample delivered to an output terminal will appear insuccessive time slots until replaced by a new coded sample delivered tothe terminal by the coder to replace the sample currently beingcirculated. Advantageously, such delay means may comprise a shiftregister, as known in the art', having its serial output connected toits serial input. The time required to shift the digits of the codedsample through the shift register is established so as not lo be greaterthan the time slot interval. Input digits will take precedence overdigits being circulated in the register. ln the absence of input digits,however, the digits present in the register will be continuouslyrecirculated through the register.

It is a feature of this invention that a plural signal coding circuitcomprise a single coder, a plurality of signal sources, first gatingmeans for connecting each. signal source in turn to the coder, aplurality of storage means, and second gating means for connecting theoutput ofthe coder to each storage means in turn.

t is another feature of this invention that the coder is connected insequence to each signal source and to each storage means for recurrentsampling intervals.

It is a further feature of this invention that the storage meanscomprises circulating delay means to store a particular coded sampleduring successive time intervals and until a new coded sample isreceived from the coder.

A complete understanding of these and other features of this inventionmay be gained from consideration ofA the following detailed description,together with the accompanying drawing, in which:

FIG. l is a schematic representation in block diagram form of atelephone system in which a signaling circuit inV accordance with thisinvention may be employed;

FIG. Z is a schematic representation of a signaling circuit employingdistinct coding means for each signal source, as known in the art;

FIG. 3 is a schematic representation of one illustrative embodiment of asignaling circuit in accordance with this invention that may be employedin the telephone system of FIG. l; and

FIG. 4 is a pulse sequence chart indicating the operaiton ofthe circuitof FIG. 3.

Turning now to the drawing, there is depicted in FIG. 1 a telephonesystem of the type disclosed in the aforementioned D. B. lames ct al.application, wherein circuits in accordance with my invention mayadvantageously be employed. In this system a plurality of subscribertelephones lt) are individually connected to subscriber lines 11 and maybe selectively connected by a switching network 12 in remoteconcentrator units A and B to a modular center C over commontransmission links 13. Control equipment 14 in the modular center C thenis operated, for example, in accordance with signals from a subscribertelephone lti at remote concentrator A to complete a connection throughthe central switches 15 to a called subscribers telephone in the sameconcentrator A, in remote concenlrator B or over trunks to other remoteconccntrators and foreign telephone systems.

Advantageously, the system may be operated on a time division multiplexbasis in which each subscribers telephone 10 is assigned a particularsampling period or time slot in a cyclicully recurreing group of timeslots; the repetitively recurring cycle of time slots is referred to asa frame. Upon each occurrence of a time slot assigned to a particularcalling subscribers telephone a sample of information is transmittedfrom his telephone through the switching network 12 to encoder 16 at theremote concentrator. The sample is transformed into a series of digitalimpulses and transmitted over the S lead of transmission line 13 to thecentral switches 15 at the modular center C. From the modular center Cthe signal sample, still in digital form, is transmitted to thereceiving subscribcrs telephone. Considering that the receivingsubscriber is located in another remote concentrator, the digital signalis transmitted over the R lead of another transmission line 13 todecoder 17 at the remote concentrator where it is restored to a voicefrequency signal and received at the receiving subscribcrs telephone inthe same time slot of a succeeding frame.

lt may be note l in this brief outline ofthe system operation that alltransmission between the remote concentrator units and the switchingcenter is accomplished in digital lll fit

form and that the equipment for translation between voice frequency anddigital signals is accomplished at the remote concentrator units.

The same transmission lines t3 are employed for talking and signaling.Thus, the various signaling tones must also be placed in digital form topermit transmission over the common links. Equipment to provide thevarious tones advantageously is located at the modular center C and isprovided in common to all remote conccntrator units, thus obviatingduplication of expensive tone generating equipment at each of the remoteconcentrator units. Such tone control equipment 18 appears in FlC. lconnected between the central control 14 and central switches 15 at themodular center C. As all signals transmitted through the modular centerC are in digital form, the variety of tones generated at the modularcenter as required in such a large scale telephone system must alsoappear in digital form.

Assume that two subscriber telephones 1t) in distinct remoteconcentrators such as A and B require ringing tone and are assigned timeslots 1 and 2, respectively. The control equipment at the switchingcenter would be activated to connect the coded ringing tone sample tothe R leg of the transmission line 13 to concentrator A during the firsttime slot and to concentrator B during the second time slot.Sin'iilarly, subscriber telephones in other remote concentrator unitsmay require ringing tone in time siots 1 and 2 or other time slots ofthe frame. whereupon the coded ringing tone samples are applied to the Rleg of each transmission line 13 to such other remote eoncentratorunits. lt is evident, therefore that a coded sample of the ringing tonemust he available during each time slot of the frame and that the codedsamples must be changed in cach successive frame to perpetuate the timedivided ringing tone.

Considering that a large number of tones must be available in such atelephone system, the duplication of coding equipment as noted in FIG.2, to process each tone, would be costly. In this arrangement a sampleof each tone 1 to n is stored in a corresponding condenser 2l duringeach time slot interval by operation of the corresponding gate 2t). Eachsample is then placed in digital form in the corresponding coder 22 andmade available at its output terminal.

It may be noted that the circuit of FIG. 2 provides a new coded sampleof each tone during each time slot of the frame. However, as indicatedhercinbefore, the system merely requires that a new coded sample of eachtone be made available in cach successive frame. It is possible,therefore, to eliminate the duplication of equipment noted in FIG. 2 byemployment of a coding circuit in accordance with my invention.

One specific illustrative embodiment of a signaling circuit inaccordance with my invention is depicted in FIG. 3 and comprises asingle coder 31), storage means 31 and sequence switches 32 and 33 atthe input and output of the coder Sti, respectively. The various tonesrcquired in the telephone system shown in FiG. l are representedspeciiicaily in FIG. 3 by ringing tone and busy tone sources, thoughother tone sources for other purposes are also provided, as indicated inFlG. 3.

The switch 32 comprises sampling gates 34 connected to cach tone source.The gates are enabled in sequence by control signals from centralcontrol 14 so as to transmit samples of each tone to the coder 30. Thesampling time has an established duration less than one time slotinterval, and each tone source is sampled once per frame interval.

The switch 33 at the coder output serves to direct the coded signalsamples sequentially to a corresponding one of the storage means 31. Theoperating .sequence of switch 32 is synchronized with the operatingsequence of switch 33. Fach partit. `ar tono source, therefore, willhave a corresponding receptacle for storage of coded samples of itsfrequency.

The sequentially operated switches 32 and 33 advantageously may compriselogic circuitry adapted to high speed operation. Gating circuitry fortransfer of vo1ce frequency signals demands greater accuracy than thatrequired for transfer of digital signals. Thus the sequence switch 32advantageously may comprise a series of bilateral transistor gates ofthe type disclosed in J. D. Iohannescn, P. B. Myers and J. E. SchwcnkerPatent No. 2,899,570, issued August 1l, 1959.

The transistors in such a gate normally afford a high impedance to thepassage of signals therethrough, but upon the application of a controlsignal to appropriate terminals, the transistors will assume a very lowimpcdance state during which signals may be transmitted therethroughwith negligible loss. The control signal is removed at the end of thesampling period, thereby restoring the high impedance condition of thetransistors. Appropriate control signals in the instant system areprovided by the central control 14 and take the form of accurately timedpulses applied to each gate in sequence.

The coder 30 acts upon each tone sample passed by the sequence switch 32and stored in condenser 35. Advantageously, an inductance is insertedbetween the sequence switch 32. and the condenser 35. as indicated inFIG. 3, to permit low loss transfer of the tone samples through thesequence switch 32 in accordance with the resonant transfer circuitdisclosed by W. D. Lewis in his Patent No. 2.936.337. issued May l0,1960.

One form of coder satisfactory for this purpose. as disclosed in theaforementioned patent to I. R. Pierce, performs comparisons of thissample with precise reference amplitude control signals receivedsuccessively' from central control 14. Each reference signal is one-halfthe amplitude of the preceding reference signal. lf a reference signalamplitude is greater than that of the sample, a zero output signalrepresentative of one binary code digit is provided. and the referencesignal is rejected. lf a reference signal amplitude is smaller' thanthat of the sample, an output signal representative ofthe other binarydigit is provided. and the reference signal is added to the succeedingreference signal. After a plurality of such trial comparisons thereference signal amplitude will closely match that of the sample. andthe coded information provided in the interim will define this finalamplitude.

Advantageously, compressor circuitry, as known in the art, may `beemployed in conjunction with the coder 3ft to reduce the amplitude ofthe signal sample. The coder circuitry may be simplified by thisexpedient, since fewer digits are required to identify a compressedsignal sample. In addition. the telephone system of FlG. l employscompressors in the common transmission paths so that compressed codedtone samples are desirable.

Coder output switch 33. for passage of digital signals in sequence. maybe of considerably simplified form and may comprise. for example. aplurality of AND logic gates 36 of types known in the art connected incommon to the coder output. A second input to each of the AND gates isconnected to the common control 14 from which pulses are applied insequence to the AND gates. The AND gate is enabled solely upon thesiniultaneous occurrence of pulses at cach of its inputs. At all othertimes the AND gate will block the passage of signals therethrough. Thusupon the simultaneous appearance of an output signal from the coder 3Gand a control signal from common control 14 at thc inputs of any one ofthe AND gates, the coder output signal will be transmitted through theselected AND gate. Coded signal samples are therefore made available atthe output of each AND gate 35 corresponding to the samples taken fromthe tone sources.

As cach tone is sampled during only one time slot in each frame. a codedsample is available at the output of each AND gate only during a singletime slot interval per frame. The telephone system depicted in FIG. l

requires that each coded tone be available for sampling during everytime slot of the frame, as described hereinbefore. Thus, in accordancewith this invention, storage means 3l are provided for assuring thatcoded samples transmitted through the coder output switch 33 are storedfor a complete frame interval and are repeated at an output terminalduring each time slot interval of the trarne. Each of the storage means31 may comprise a shift register of a type known in the art whichregisters will circulate the coded signal samples and have the samplesavailable at corresponding outputs in serial form at the beginning ofeach time slot. The output of each shift register is connected to itsinput so that once a coded sample is inserted in a shift register itwill continue to circulate therethrough until a new coded sample isreceived.

A shift register satisfactory for this purpose` as shown for example inHigh-Speed Computing Devices, Engineering Research Associates, lne.,page 299, FIGS. 13-25, McGraw-Hill Book Company, Inc.. New York. 1950.may comprise a serics of bistable flip-dop circuits. Application of eachdigit of the coded word to the first flip-flop in conjunction with ashift pulse from the central control 14 will cause the flip-flop toreverse its state or remain in the current state. dependent upon thetype of digit signal received. Similarly. the state of each dip-flop.upon application ol' the shift pulse. will he determined by the currentstate of thc preceding flip-flop.

The digit signal emanating from the final stage of each shift registeris transmitted to a corresponding output gate 39 and through acorresponding logic circuit 37 to the first register stage to permit thecontinuous circulation of the coded signal sample. Upon receipt by oneof the AND gates in switch 33 of a control signal indicative of theavailability of a new coded sample for the corresponding outputterminal, the logic circuit 37 connected between the output and input ofthe corresponding shift register is activated to inhibit passage ofdigit signals therethrough. Upon completion of storage in the shiftregister of the new coded signal sample. the control signal is removedfrom inhibit circuit 37, and the digit signals of this coded sample arerecirculnted in the register. Inhibit logic circuits suitable for ,useas logic circuit 37u11 known in the art.

Timing of the control operations is established such that at thebeginning of each time slot interval the first digit of a coded sampleis available at each corresponding output gate 39. The time slotinterval is of sufficient length to accommodate an entire coded tonesample. The timing for this circuit operation may best be understood byreference to the pulse sequence chart indicated in FIG. 4. The interval4l represents a time slot sufficient to include eight binary digitpulses or bits, the length selected for the telephone system depicted inFIG. l. An eight bit word is more than adequate to define any discretefrequency level. Low frequency tone signals in turn may be defined by asfew as two code bits, thc. balance of the time slot interval acting as aguard space. During this guard space, for example, storage condenser 35may be grounded to remove all traces of one signal sample prior toarrival of the next sample.

Sampling in the system of FIG. l. as noted hereinbcfore. may occur at aneight ltiloeycle repetition rate. Thus the frame interval, during whichcach tone is sampled in sequence is im@ of a second. Samplingy isinitiated by a pulse 42 from central control 14 which enables gate 34connected to the ringing tone source, FIG. 3, at the beginning of a timeslot 4l. During this interval a sample of the ringing tone istransmitted to the coder 3ft. The sample is then placed in coded formassuming a contiguration such as shown at the output of the coder 30 inFIG. 3. Concurrent with the availability of the first binary digit orhit of the coded word. a signal 43 from the central control 14 enablesAND gate 35 of sequence switch 33 corresponding to the ringing tonesource. The

signal 43 is also transmitted to the corresponding logic circuit 37 toinhibit the transfer of the coded word then present in shift register 38from. continuing to circulate therein. The new coded word is thenshifted serially into the shift register 38 and is available at terminal39 at the beginning of the next time slot 4l.

The same sequence of operations is then repeated for each of theremaining tones in order, control pulses 44 and 45 enabling the gatescorresponding to the busy tone source and pulses 46 and 47 operating thegates corresponding to the next tone source. Upon completion of samplingof all available tones, switches 32 and 33 will be maintained inactiveuntil a complete frame interval has elapsed, whereupon the tones areagain sampled sequentially and processed through the circuit to replacethe coded samples stored during the previous frame. The length of aframe in the telephone system depicted in FIG. l may be established at25 time slots, as noted hcreinbefore, such that up to 2S tones could bemade available with the instant invention during cach frame, utilizingthe single coder 31.

While my invention has been disclosed with reference to particular logiccircuitry in conjunction with a coder and shift register elements, itmay be noted that various component substitutions are permitted. Forexample, a coder of the cathode ray tube type, as disclosed in W. M.Goodall Patent 2,616,060, issucd October 28, 1952, may be utilized inplace of the coder described liereinbefore. Also, the shift registerstorage means may be replaced by various forms of circulating delaymeans, as known in the art, to provide the essential storage operationat the coder output.

lt is to be understood that the above-described arrangements areillustrative of the application of the principles of the invention.Numerous other arrangements may be devised by those skilled in the artwithout departing from the spirit and scope of the invention.

What is claimed is:

1. In a time division switching system, the combination for providingdiscrete coded samples of a plurality of tone signal frequencies to atransmission line corprising a plurality of distinct signal frequencysources, a coder, first gating means for connecting any one of saidsources to said coder, means for enabling said first gating means toconnect each of said sources to said coder in a particular time slot ina repeated cycle of said time slots, a plurality of storage means,second gating means for connecting the output of said coder to any oneof said storage means, means for enabling said second gating means toconnect said coder to each of said storage means in the time slotreserved for a corresponding one of said sources, third gating means forselectively connecting the output of any one of said storage means tosaid transmission line in any one of said time slots, and means forcontinuously circulating the coded signal samples provided by said codcrthrough said storage means.

2. In a time division multiplex communication system employing pulsecode modulated transmission signals, the combination for makingavailable a plurality of coded tones for selective sampling by saidsystem comprising a coder, first switching means for selectivelyconnecting a plurality of different frequency tone sources to saidcoder, digital storage means corresponding to each of said tone sources,second switching means for selectively connecting said coder to saidstorage means, and means for operating said first and second switchingmeans in synchronism to stored coded samples of tones from said sourcesin said corresponding digital storage means.

3. In a time division multiplex communication system, the combination inaccordance with claim 2 further comprising means for continuouslycirculating said coded samples of tones in said digital storage means.

4. ln a time division communication system, the combination inaccordance with claim 3 further comprising means for blocking therecirculation of said coded samples of tones in said digital storagemeans and means for enabling said blocking means responsive to receiptin said digital storage means of one of said coded samples of tones.

S. A tone conversion circuit comprising a plurality of distinct tonesources, pulse code modulating means, first means for directing samplesof tones from each of said tone sources to said pulse code modulatingmeans in sequence, a plurality of pulse registering means, each capableof storing a tone sample in coded form, second means for directing atone sample in coded form from said pulse code modulating means to oneof said pulse registering means corresponding to said source providingsaid tone sample, and means for continuously circulating said codedsample in said pulse registering means.

6. A tone conversion circuit in accordance with claim S furthercomprising means for synchronizing the operation of said first andsecond sample directing means.

7. A tone conversion circuit in accordance with claim 5 furthercomprising means for blocking the recirculation of a coded sample insaid pulse registering means upon receipt at said pulse registeringmeans of a new coded sample.

8. ln a communication system, apparatus for providing each of aplurality of signaling tones in digital form during each of a successionof time slot intervals in a frame interval comprising means for codingtone frequency samples in digital form, means for directing samples of aplurality of tone frequencies to said coding means in successive timeslot intervals of a recurrent frame of time slot intervals, means forstoring coded samples of corresponding tone frequencies, means operatcdin synchronism with said directing means for directing coded samplesfrom said coding means to said corresponding storage means, means forrccirculating said coded samples through said storage means, and meansfor inhibiting such recirculation upon receipt of sub` sequent codedsamples at said storage means.

9. A tone conversion circuit comprising a plurality of distinct tonesources, cooling means, means for sampling the tones from said tonesources on a sclcctive basis, means for applying said tone samples insequence to said coder, circulating delay means corresponding to each ofsaid tone sources. means for selectively connecting the output of saidcoder to each of said delay means in sequence, and means forsynchronizing the operation of said sampling means and said connectingmeans.

10. A tone conversion circuit in accordance with claim 9 furthercomprising means for inhibiting the reentry of information stored insaid circulating delay means and means including said connecting meansfor activating said inhibiting means.

l1. A signal conversion circuit comprising a plurality of distinctsignal frequency sources, a coder, first gating means for connecting anyone of said sources to said coder, means for enabling said first gatingmeans to connect each of said sources individually and in time sequenceto said coder, a plurality of storage means, second gating means, meansfor enabling said second gating means to connect said coder to each ofsaid storage means individually and in time sequence, and means forcontinuously circulating through said storage means the coded signalsamples provided by said coder.

12. A signal conversion circuit for placing a plurality of distinctfrequency signals in digital form comprising a coder, a plurality ofsignal sources, first means for connecting each of said signal sourcesto said coder during distinct time intervals, storage meanscorresponding to each of said signal sources, and second means forconnecting said coder to each of said storage means during correspondingdistinct time intervals.

13. A signal conversion circuit in accordance with claim l2 wherein saidstorage means comprises a plurality of shift registers and furthercomprising logic means connected between the output and input of each ofsaid shift registers, said logic means being responsive to the receiptof a coded sample to inhibit the recirculation of a coded samplecurrently stored in said shift register.

14. A signal conversion circuit in accordance with claim 12 wherein saidrst connecting means comprises a plurality of sampling gates connectedto corresponding ones of said signal sources and said second connectingmeans comprises a plurality of coincidence gates connected tocorresponding ones of said storage means.

15. A signal conversion circuit in accordance with claim 14 and furthercomprising means for enabling said sampling gates and said coincidencegates, respectively, in sequence and in synchronism with each other.

i6. In a time division multiplex switching system, a first group oftransmission paths, a second group of transmission paths, a transmissionmedium between and com` mon to said paths, a supervisory tone source, arst group of transmission gates each interposed between a respectivefirst group transmission path and' the common medium, a second group oftransmission gates, one of the second-group transmission gates being atone transmission gate interposed between said tone source and thecommon medium, other of the second-group transmission gates being eachinterposed between a respective secondgroup transmission path and thecommon medium, means for applying coincident trains of enabling pulsesto a predetermined pair of first-group and second-group trans-References Cited by the Examiner The following references, cited by theExaminer, are of record in the patented file of this patent or theoriginal patent.

UNITED STATES PATENTS 2,451,044 10/1948 Pierce 179-15 2,520,185 8/1950Van Mierlo 179-15 2,830,120 4/1958 Trousdale 179*18 2,877,306 3/1959Baker 179-18 2,917,583 12/1959 Burton 179-18 2,929,879 3/1960 Jacobaeuset al. 179-l8 2,936,338 5/1960 James et al 179-18 ROBERT H. ROSE,Primary Examiner.

WALTER L. LYNDE, Examiner.

